We grow up hearing about acid rain, but what about other forms of acidic precipitation — do they even exist?

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One form, acid snow, certainly does exist. Acid snow is as natural as acid rain in our post-industrial revolution world and it poses an unusual threat. Let's delve into the properties of acid snow, along with its dangerous backlash when Spring comes and temperatures rise.

The top image is an altered form of an image by day-light on deviantART.

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How is acidic precipitation created?

Normal rain is already somewhat acidic, due to the absorption of carbon dioxide in the atmosphere. The carbon dioxide in the air becomes carbonic acid, a weak acid, leading to a pH of approximately 5.6 for most rainfall across the world.

Acid precipitation is created when industrial pollutants like sulfur oxide and nitrogen oxide react with water in the atmosphere. Upon reaction with water, nitric acid and sulfuric acid forms, and this decreases the pH of the ensuing precipitation.

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Both sulfur oxide and nitrogen oxide are pollutants created in the process of fuel combustion, with nitrogen and sulfur oxide also released through the use of coal to produce electricity.

Less acidic than acid rain
Acid snow is slightly less acidic than acid rain, due to the crystalline structure of ice as it falls. The pH of acid snow ranges from 4.4 to 4.6, while acid rain varies from a pH of 4.2 to 4.4.

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Roughly thirty percent of land on the planet is covered by snow during the Winter, with cycles of thawing and re-freezing occurring throughout the Winter months as temperatures rise and fall.

These freeze-thaw cycles give an opportunity for the sulfuric and nitric acid in acid snow to pool in one area, leading to a very dangerous environmental phenomenon known as the acid pulse, once Spring comes and melts the snow on the ground.

The Acid Pulse
As longstanding snow starts to melt, the pH of the initial stream of melt-water is often between 3.4 and 3.6, with this rush of water into the soil and nearby lakes and streams called an acid pulse. This jolt of acid damages soil and bodies of water during the early Spring, a critical growth time for plants and wildlife. The acid pulse decreases the pH of surrounding streams and small lakes by as much as one pH unit. A single pH unit change may not sound substantial, but pH is measured on a logarithmic scale, so a one unit decrease is a ten-fold increase in acid content.

The acidic runoff is of particular danger to eggs in the soil and water; eggs preparing to hatch with the change of seasons. The pulse only lasts for a few days before the majority of the acid content is exhausted and the pH returns to a normal level.

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Salamander populations observed in Colorado experienced a 70% decline in viability due to damage to the eggs from an acid pulse. Monitoring of acid snow in New York State showed that an acid pulse killed over fifty-thousand recently hatched trout in a single lake.

Not new, but needs to be monitored
Acidic snow is not a new phenomenon, but it is one with a hidden backlash when Spring comes. This backlash calls for increased monitoring of the effect of acid snow on wildlife as it falls, along with the sudden change in water and soil pH that comes along with release of its acid pulse in the Spring.